Method and apparatus for indirectly drying and preheating fine material

ABSTRACT

A method and apparatus is disclosed for indirectly drying and preheating fine material, in particular, moisture containing coal or the like, comprises, providing a rotary drum which is mounted for rotation and has a plurality of heating tubes extending therethrough, supplying heated cooling gas from a dry coke cooling system to the heating tubes, supplying the moist material to the drum, adjacent the bottom thereof, removing the material from the drum after it has been predried and preheated, and returning or recycling a portion of the predried and preheated material to the rotary drum. The method and apparatus utilizes the heated cooling gas used to cool dry coke in a dry coke cooling plant from high temperatures to below 200° C. It has been found that such heated cooling gas for a selected amount of dry coke is useful either in a series of more than one rotary drum or, to process moist material in a single rotary drum and also generate steam as a byproduct.

This is a division of application Ser. No. 295,496 filed Aug. 24, 1981,U.S. Pat. No. 4,392,823.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to the drying of moist material ingeneral, such as, bituminous and sub-bituminous coal, peat, wood, oilshale, ores, limestone and the like and, in particular, to a new anduseful method and apparatus for preheating and drying such moistmaterial using a rotary drum having heating tubes therethrough which aresupplied with heated cooling gas from a dry coke cooling plant.

A coal mud drier is known from German Offenlegungsscrift No. 28 44 075,which is equipped with a hot gas drying system in a rotary drierfurnace, in which a number of tubes is rigidly mounted in thelongitudinal direction of the rotary drum of the furnace. Heating gasesare continuously circulated through these tubes while waste gases,predominantly only enriched with steam, escape from the interior of thedrum into the open atmosphere. To produce the heating gases, a separateburner is needed which must be operated with rich gases.

Such rotary drum driers are mostly disposed with a slight inclinationand the moist fine material is fed in at the highest point of the drumwhile the preheated material is discharged at the lowest point. Properstructures are provided on the inside surface of the rotary drum bywhich the drying material migrating down between the heating tubes ofthe nests and accumulating on the drum bottom is lifted by the rotationof the drum and dropped again from above on the tube nests. With theslight inclination of the rotary drum, the fine material fed in at oneend gradually passes to the other end where it is discharged. During itstravel from the higher to the lower end, the fine material is repeatedlyraised to the tube nests to flow down between the tubes and be dried andpreheated.

To insure a trouble-free operation of such rotary drum driers, certainminimum requirements must be imposed on the charged material. Forexample, while drying coal, the moisture content of the charged coalmust not affect the fluidity of the material charged to the tube nests.Otherwise, congestions and deposits of coal between the tubes and thus areduced efficiency of the drier can be expected. In such cases, a remedymay be to enlarge the spacing between the tubes, for example, from 20 mmto 40 mm. However, this substantially reduces the heat exchange surfaceareas and thus again the efficiency of the drier. What is to be soughtis rather to enlarge the heat exchange surface area by reducing theclearances for the passage of the fine material and thus to increase thedrier efficiency. This particularly applies to the design of driers witha high throughput capacity.

Further difficulties arise during the operation of such drum driers withthe removal of the produced vapors.

In these driers, the vapors are taken off toward the discharge end ofthe rotary drum through an opening in the top portion above the heatingtube nest. Since for constructional reasons, this opening cannot be toolarge, the produced vapors entrain much fine dust and an expensive dustseparation must be provided outside the rotary drier.

SUMMARY OF THE INVENTION

The present invention is directed to an economical method of indirectdrying and preheating of fine material, requiring no burning of richgases, and improving the operation and increasing the efficiency of theindirectly heated drum drier by insuring a uniform and trouble-freepassage of the fine material therethrough and improving the evacuationof vapors.

Accordingly, an object of the present invention is to provide a methodof indirectly drying and preheating fine material, such as bituminousand sub-bituminous coal, peat, wood, oil shale, ores or limestone, inwhich the fine material is dried and preheated by means of heating tubesthrough which gases are continuously circulated, and includingconducting heated cooling gas from a dry coke cooling system, after dusthas been separated from such heated cooling gas, as heating gas throughthe heating tubes of a rotary drum drier, and recycling a portion of thepreheated and dried fine material to the rotary drum.

While applying these inventive operating steps, it has been found that,surprisingly, the heat absorbed during the dry coke cooling process bythe circulating gases is sufficient, in every instance, for drying thecoal and preheating it to about 200° C. An excess heat of about 50% evenremains as a balance.

Another object of the invention is to provide such a method wherein theheated cooling gas from the dry coke cooling system is initially cooledto between 500° C. and 600° C. in a steam generator, before the heatedcooling gas is supplied to the heating tubes of the drier.

This preliminary cooling of the gases is particularly beneficial to thematerial of the heating tubes. Also, it favors steam generation, if thefeed water from the secondary cooler which is provided upstream of thelocation where the cooling gases enter the dry coke cooling system, issupplied directly to the steam generator.

In cases where the dry coke cooling system is designed almost entirelyfor steam generation, it has proved particularly advantageous to conductthe steam produced during the cooling of coke in a dry coke coolingplant through the heating tubes of the rotary drum drier.

In this connection, it may be particularly economical to employ pressuresteam pipes for the tube nests. This further calls for using, for thecoal drying, a steam produced during the dry coke cooling. In such acase, of course, the equipment for circulating and controlling theheating gases is omitted. Only a steam supply line and a condensatedrain are needed, and the drying process will be controlled in a simpleway by the temperature of the steam.

A still further object of the invention is to provide an apparatus forcarrying out the method which comprises an outer rotary drum having aplurality of heating tubes in heating tube nests extending therethrough,feed and discharge mechanisms for moist and preheated fine materialprovided at a front side end of the drum, and devices for taking driedvapors from the drum, wherein, the discharge mechanism for the preheatedand dried material is connected, at a location outside the rotary drum,to a return line for initially providing such moist material to therotary drum through a feed mechanism.

The inventive design makes it possible to mix any adjustable amount ofpreheated fine material outside the rotary drum drier proper with thecharged moist material and to dry and preheat the moist and cold chargeby direct heat exchange between solid and solid.

An evaporation bin having an offtake for vapors and equipped with amixing screw for the moist and the preheated fine material is providedupstream of the feed mechanism.

The inventive evaporation bin makes it possible to evacuate substantialamounts of water vapor already prior to feeding the material to therotary drum, so that the amount of vapors to be removed from the rotarydrum is reduced. In consequence, the amount of dust entrained from therotary drum drier with the residual vapors is also less, as compared tothe prior art methods.

The invention further provides a feed mechanism designed as a screwconveyor extending into the lower zone of the rotary drum at one frontor upstream side thereof. It is particularly advantageous if the feedmechanism projects into the rotary drum below the stationary heatingtubes, unlike the prior art which supplies the material from above.

Due to such a provision, the coal fed in, mixes with the predried coalwhich has already passed once or several times from above, between thetubes to the bottom of the drum, before it is raised for the first timeto the heating tube nest above. Congestions by deposits of coal betweenthe tubes are thereby avoided and the product can travel through therotary drum driver without trouble.

As to the relocation of the feed mechanism into the lower zone of thedrum, it is advantageous to provide offtakes for the drier vapors ateach of the front sides or ends of the drum, in the upper zone of therotary drum above the heating tube nest. While in the prior art, themoist material has been fed in at one front side, in the upper zone ofthe rotary drum and the vapors could be evacuated only at the oppositeside, the inventive evacuation of vapors at both sides offers theadvantage that the speed at which the produced vapors are taken off isreduced to one-half so that less dust is also entrained.

Since the fluidity of the material to be dried gradually increases inthe travel direction thereof and the possibility of congestionsdecreases accordingly toward the discharge end, it is provided, inaccordance with the invention, that the diameter of the tubes of theheating tube nests increases stepwise in the direction of travel of thedrying material within the rotary drum. It is further advantageous toreduce the diameter of the tubes only in the zone where the finematerial is fed into the rotary drum initially.

As to the design of the tube nests, the invention provides, in addition,a hair pin construction, with the tubes of the nest projecting into therotary drum from the two stationary drum heads. This design is alsofavorable in that it makes it possible to provide, in the rotary drum, alarger number of tubes and reduced spacing at the side where thepreheated fine material is discharged, than at the feed side.

To seal the rotary drum against the stationary drum heads, the inventionprovides that the seals between the drum and the heads include a smoothring surface on the rotating side, and a unilateral labyrinth gland onthe opposite, stationary side. Advantageously, a ring conduit isprovided in a recess of the labyrinth gland, for supplying a sealinggas. While in the prior art, the seal between the rotary part of thedrum and the stationary housing has been designed as a simple slidingseal, metal on metal, the invention provides a unilateral labyrinthgland and a supply of a sealing gas, advantageously superheated steam.

The inventive design, unlike with a simple sliding seal, makes itpossible to substantially reduce the contact pressure and thus thefriction losses. The sealing gas flowing toward the interior of the druminsures, in addition, that no coal dust can deposit in the seal gap.

Further, the seals may be pressed into contact with the drum throughhydraulically operated cylinders provided at the periphery, and thecontact pressure may automatically be kept constant through thehydraulic system. In this way, even a low contact pressure can be keptconstant and still some wobble of the drum and longitudinal dilatationdue to greater temperature variations, for example, are compensated for.

In certain cases, it has proved advantageous in the construction andoperation of the rotary drum drier, instead of an immovable mounting ofthe heating tube nests on the drum heads, to mount the nests forrotation through a separate drive, in the direction of the drumrotation, or opposite thereto.

This primarily reduces the risk of congestion and coal deposits betweenthe tubes. Also, the coal may remain longer at the heating tubes, sothat more favorable conditions are created for the convective heattransfer from the heating surfaces to the coal.

In accordance with the invention, with the same direction of rotation,it is advisable to provide a double to fourfold speed of the drum,relative to the inside heating tube nest.

Another object of the invention is to provide such a device which issimple in design, rugged in construction and economical to manufacture.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Drawings:

FIG. 1 is a diagrammatical illustration of a system and apparatusaccording to the invention for practicing the inventive method;

FIG. 2 is a diagrammatical illustration of an apparatus for practicingthe invention which combines a rotary drum drier with a dry coke coolingsystem;

FIG. 3 is a view similar to FIG. 2 of another embodiment of theinvention; and

FIG. 4 is a side fragmentary sectional view, partially in elevation, ofa labyrinth seal, used in the apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to the drawings in particular, the invention embodied therein inFIG. 1, shows a bin 1 for the moist fine material to be dried, which isconnected, through an outlet 2, a conveyor-type weigher 3, and adelivery means 4, to a mixing screw 6 and an evaporation bin 7. Beneaththe evaporation bin 7, a screw conveyor 9 is provided projecting into arotary drum 10 from the front side thereof, below a nest 30 of heatingtubes.

Mixing screw 6, in addition to being connected to the moist coal supply1 to 4, is also connected to the discharge lock 18 of rotary drum 10through a return line 5. Return line 5 is embodied, for example, byenclosed screw conveyors or chain conveyors, known per se.

The movable parts of rotary drum 10 are associated with a drive 11 andtwo bearings 12 and 13, while the two drum heads 14 and 15 arestationary or rotatable and are sealed against rotary drum 10 by meansof seals 16 and 17.

Heating tube nests 30 are fixedly anchored to drum heads 14 and 15 andextend through the drum in the axial direction thereof, from one drumhead to the other. For this purpose, the heads 14 and 15 are fixed andthe heating tube nests 30 are either fixed, or mechanically connected toa schematically shown drive means 15a for rotating, to achieve therelative motion between the tubes and the rotary drum 10.

Turning to FIG. 4, the labyrinth seal is shown which comprises aunilateral labyrinth 16a which defines at least one ring-shaped chamber16b which can be supplied with fluid, such as steam, through an opening16c. The labyrinth seal cooperates with a smooth surface 10a of drum 10.

As shown in FIG. 1, at least one of the heating tubes 30a increases indiameter, in stepwise fashion, along the travel direction shown by thearrow in rotary drum 10. In particular, tube 30a may increase by asingle step from a small diameter portion connected to the head 14, andadjacent the moist material inlet mechanism 9.

The heating fluid, which is superheated steam as a rule, is conductedthrough a connection 20 to drum head 15 and therefrom into the interiorof heating tubes 30. A condensate drain 21 is provided at the oppositehead 14.

To evacuate the vapors produced during the drying process, vaporofftakes 22 and 23 are provided above heating tube nests 30, at bothfront sides of rotary drum 10, which are connected to a cooler 24.

The vapors which have already been produced in evaporation bin 7 arealso directed to cooler 24 through a line 8. The cooler has connections25 and 26 for supplying and discharging cooling water, and a condensatedrain line 28 for the water from the process of drying.

A connection 27 makes it possible to vent cooler 24, or to take offgases therefrom by suction.

EXAMPLE OF OPERATION OF A ROTARY DRUM DRIER

As shown in FIG. 1, 80 ton/hour of fine coal having a moisture contentof 12%, are delivered from moist coal bin 1 for drying. In mixing screw6, this moist coal is also mixed with 80 t/h of already dried coal whichhas been preheated to 200° C., and the mixture is supplied intoevaporation bin 7. With an average mixture temperature of about 75° C.,approximately 2.7 t/h of vapors escape from bin 7, and they areconducted to indirect cooler 24. An amount of 157.5 t/h of coal is thusfed into the rotary drum by screw conveyor 9, having a moisture contentof about 4.4%.

While with a moisture content of 12% of the fine coal, congestion,clogging and trouble may very easily occur in bins and conveying meansdue to poor fluidity, such problems hardly arise any longer with a coalhaving a moisture content of less than 5%. Not even at an unfavorablerange of the grain sizes. If such a coal is, in addition, continuouslymixed in the rotary drum with the coal which has already once passed bythe heating tubes, it is ensured that the coal mixture charged fromabove onto the heating tube nests will flow down between the tubeswithout problems.

During drying in the rotary drum 10, further about 6.9 t/h of watervapor escapes from the coal mixture and are directed to the cooler, sothat at the end of the drum, about 150.4 t/h of preheated coal aredischarged having a temperature of approximately 200° C., of which 80t/h are recycled by conveying means 5 to mixing screw 6, and 70.4 t/hare removed through discharge line 19 to be supplied by a suitable meansof transportation, to the coke ovens.

Other examples of the invention are illustrated in FIGS. 2 and 3. Inthese figures, a dry coke cooling plant or system is shown at 100 forthe entry of hot coke at 101 and the withdrawal of the cooled coke at102.

FIG. 2 shows a system whereby a plurality of rotary drums 110 can beserviced by the heated cooling gases of a single coke cooler plant 100.In FIG. 3, a single rotary drum 110 is provided with gas, with aremainder of the heat in the heated cooling gas being utilized forgenerating steam in a steam generator 130.

The following listing of elements and their corresponding referencenumerals are provided for easy understanding of the systems illustratedin FIGS. 2 and 3. All elements are the same for both embodiments exceptwhere indicated to be specific to either FIG. 2 or FIG. 3 alone.

100--dry coke cooler plant

101--entrance hot coke

102--exit cooled coke

109--entrance moist coal

119--exit preheated coal

110--rotary drum drier

120--entrance hot gas

121--exit hot gas (to become cooling gas for 100)

122--exit vapors

127--hot cooling gas from the dry coke cooler

128--entrance hot gas for a second drier (not shown) connected inparallel (FIG. 2)

124--cooling gas from second drier (FIG. 2)

125--blower for cooling gas

126--return of cooling gas to dry coke cooler

130--steam generator (FIG. 3)

131--steam cylinder (FIG. 3)

132--feedwater supply secondary cooler (FIG. 3)

133--secondary cooler (FIG. 3)

134 to

137--steam circulation (FIG. 3)

The following table illustrates the operation of the apparatus shown inFIGS. 2 and 3, respectively, keyed into reference numerals of thesefigures.

                  TABLE                                                           ______________________________________                                        (Amounts shown PER HOUR)                                                      REF. NUMBER    FIG. 2       FIG. 3                                            ______________________________________                                                       Coke         Coke                                              101            60 t         60 t                                                             t.sub.1 = 1050° C.                                                                  t.sub.1 = 1050° C.                         102            t.sub.2 200° C.                                                                     t.sub.2 200° C.                                           Cooling Gas  Cooling Gas                                       127            95,000 m.sub.n.sup.3                                                                       95,000 m.sub.n.sup.3                                             t.sub.1 = 750° C.                                                                   t.sub.1 = 750° C.                          126            t.sub.2 = 150° C.                                                                   t.sub.2 = 150° C.                          120/128        47,500 m.sub.n.sup.2                                                                       95,000 m.sub.n.sup.3                                             Coal per Drier                                                                             Coal                                              109            80 t         90 t                                                             t.sub.1 = ca. 10° C.                                                                t.sub.1 = ca. 10° C.                                      10% moisture 10% moisture                                      119            72 t         81 t                                                             t.sub.2 = 220° C.                                                                   t.sub.2 = 220° C.                                         Vapors       Vapors                                            122            8 t each drier                                                                             9 t                                                                           Steam                                             137                         ca. 14.5 t                                                                    p = 13 bar                                                                    t = 320° C.                                ______________________________________                                    

The examples to FIGS. 2 and 3 show that it is possible, by means of theheat amounts removed from the dry coke cooler 100, to operatesimultaneously two rotary drum driers connected in parallel, asindicated in FIG. 2. As is evident from example or FIG. 2, while cooling60 t of coke from 1,050° C. to less than 200° C., it would be possibleto preheat about 160 t of coal to 200° C. This is practicable if, in acooling plant, only a part of the coke production is cooled in such adry coke cooler.

In the case shown in FIG. 3, the 90 t of coal to be preheated are anadequate amount for producing 60 t of coke. The excess heat from thecooler is used for generating steam in excess in generator 130.

It should be further noted that the inventive solution shown in FIG. 2,with the supply of the hot cooling gases from the dry coke cooler intopreheating tubes of the rotary drum drier, is the most simple andinexpensive one, since no additional heat exchanger is needed for thesteam generation. Should the dry coke cooler and the coal drying plantbe provided at locations which are widely spaced apart, it may be moreadvantageous to first generate steam indirectly, and then to supply thissteam into the heating tubes of the drier, since over larger distances,the transportation of this steam in pipes of relatively smaller diameteris simpler than the supply of the hot cooling gases through pipes havinga very large diameter.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A method of indirectly drying and preheating fine material, comprising:providing a rotary drum having a plurality of heating tubes extending therethrough; supplying heated cooling gases from a dry coke cooling system, from which dust has been separated, to the heating tubes; feeding the fine material to be dried and preheated to the rotary drum for preheating and drying the fine material; removing the dried and preheated fine material from the rotary drum; and recycling a portion of the dried and preheated material to the rotary drum.
 2. A method, according to claim 1, comprising, initially cooling the heated cooling gases from the dry coke cooling system in a steam generator, to a temperature of between about 500° C. and 600° C., and thereafter, directing the cooling gases to the heating tubes.
 3. A method, according to claim 1, including producing steam from the heat of the heated cooling gases of the dry coke cooling plant and directing the thus produced steam through the heating tubes.
 4. A method, according to claim 1, including rotating the heating tubes in the rotary drum in the same direction as a rotation of the rotary drum but at a speed between one-half and one-quarter that of the rotary drum.
 5. A method, according to claim 1, including rotating the heating tubes in a direction opposite the rotation of the rotary drum.
 6. A method, according to claim 1, including maintaining the heating tubes in a fixed position with respect to the rotation of the rotary drum. 